1. Field of the Invention
This invention relates to an .alpha.-corundum composition of exceptionally high purity of at least 99.95 weight percent. This invention is also directed to a method of forming such a highly pure .alpha.-corundum composition by heating alumina having an alkali content between 0.05 and 0.60 and alumina content of more than 95 weight percent with silica and additional alkali if desired, in an electric arc furnace, cooling the resultant melt, comminuting the same and separating the so formed glass phase from an .alpha.-corundum phase by chemical and/or mechanical methods.
2. Discussion of the Prior Art
Electric furnace corundums are generally manufactured by the fusion of raw materials rich in aluminum oxide in the electric furnace. If it is desired to produce corundums of high aluminum oxide content, which are known as noble corundums, aluminas known as Bayer aluminas are used in most cases as the raw material. Such aluminas, which are chemically produced by the alkaline treatment of bauxite, yield corundums upon melting which consist of aluminum oxide of a purity of 99.6% by weight, but a portion, up to 7% of the weight of the aluminum oxide, consists of undesirable .beta.-corundum.
In recent times, however, corundums have been required for ceramics and special purpose ceramics, such as ceramics for cataphoresis, bioceramics or electroceramics, or for certain special abrasive applications, such as the finishing of semiconductor material, which contain 99.9% pure aluminum oxide by weight, and are largely free of .beta.-corundum. In the applications named above, the principal impurities, such as alkalies and silicon dioxide, as well as .beta.-corundum, are undesirable.
For the production of noble corundums of an especially high purity and a high .alpha.-corundum content it has been considered necessary to use extremely pure, low-alkali aluminas as the raw material for fusion. The alkali content was to amount to no more than 0.1% by weight in order to arrive at noble corundums of up 99.8 wt.-% Al.sub.2 O.sub.3.
The production of such pure raw materials, however, involves a considerable technical investment, which makes the raw materials and hence the end product extraordinarily expensive. It is furthermore disadvantageous that, in general, one does not exceed in obtaining a purity better than approximately 99.8% by weight. Another disadvantage is that the quality of the corundum produced always depends on the quality of the Al.sub.2 O.sub.3 charge put into the furnace in each case. In addition, impurities can intrude during the melting and crushing process to further diminish the purity below the desired level.